The present invention relates to a power transmission device provided with the function of a torque limiter. The present invention can be widely used in compressors, industrial machinery, etc.
Many kinds of this type of power transmission devices have been proposed. One is disclosed in Japanese Patent Laid-Open Publication No.311399/1998, which will be explained referring to FIGS. 3 to 5.
The power transmission device taught by Japanese Patent Laid-Open publication No.311399/1998 comprises a cylindrical rotor 22 made of ferro-alloy inserted in a pulley 21. The rotor 22 is welded to the pulley 21. A ball bearing 23 is disposed inside of the rotor 22. The rotor 22 is supported to be rotatable by a cylindrical protrusion 25a of a front housing 25 of a compressor (driven device) 24 through the ball bearing 23. A hub 27 operating as a driven rotary member is inserted in a rotary shaft 26 of the compressor 24. The hub 27 has a substantially cylindrical shape as shown in FIG. 5.
The hub 27 is splined to the rotary shaft 26. A ring-shaped stopper plate 28 is welded to the hub 27. A bolt 29 is inserted in the central hole of the stopper plate 28 to be threaded into a screw hole of the rotary shaft 26, thereby fixing the hub 27 to the rotary shaft 26. The stopper plate 28 locates the hub 27 relative to the rotary shaft 26. Thus, the hub 27 can rotate integrally with the shaft 26.
The hub 27 is provided with a cylindrical portion 27c at its one end distant from the compressor 24. The cylindrical portion 27c is provided with an externally threaded portion 27b.The hub 27 is provided with a cylindrical portion 27d adjacent the cylindrical portion 27c. The outer diameter of the cylindrical portion 27d is larger than that of the cylindrical portion 27c. The periphery of the cylindrical portion 27d is partially cut away to form a pair of parallel flat chord portions 27e. The hub 27 is provided with a flange portion 27a adjacent the cylindrical portion 27d. The outer diameter of the flange portion 27a is larger than those of the cylindrical portions 27c and 27d. 
Three fragmentary-annular-ring-shaped convex segments 27f are formed on the periphery of the end face of the flange portion 27a close to the cylindrical portion 27d. The end face of the flange portion 27a close to the cylindrical portion 27d forms a concave segment 27g except at the fragmentary-annular-ring-shaped convex segments 27f. A cylindrical portion 27h is formed on the end face of the flange portion 27a close to the compressor 24. The outer diameter of the cylindrical portion 27h is small enough to leave a relatively large space between its outer surface and the inner surface of the cylindrical protrusion 25a of the front housing 25.
The power transmission device comprises a cylindrical outer holder 30 made of ferro-alloy. The outer holder 30 is provided with three radially outwardly protruding lugs 30a. The outer holder 30 is fixed to the pulley 21 by three bolts 37 penetrating the lugs 30a. The power transmission device comprises a cylindrical inner holder 31 made of a ferro-alloy such as stainless steel. The inner holder 31 is provided with a radially inwardly protruding flange 31a. 
A rubber ring 32 is disposed between the outer holder 30 and the inner holder 31 to be seized by them. Therefore, torque is transmitted from the pulley 21 through outer holder 30 and the rubber ring 32 to the inner holder 31. The pulley 21, the rotor 22, the outer holder 30, the rubber ring 32 and the inner holder 31 constitute a driving rotary member rotating integrally.
The flange 31a of the inner holder 31 fits snugly on the cylindrical portion 27d of the hub 27 to constitute a driving friction member. A washer 33 is disposed between the flange 31a of the inner holder 31and the fragmentary-annular-ring-shaped convex segments 27f of the hub 27 to constitute an intermediate friction member.
The flange portion 27a of the hub 27 constitutes a first driven friction member. The end faces of the fragmentary-annular-ring-shaped con vexes 27f are coated with fluororesin coating material to reduce and stabilize the friction coefficient xcexc1 of the abutting surfaces between the fragmentary-annular-ring-shaped con vexes 27f and the washer 33. The fluororesin coating material is an electrical insulating material that protects the fragmentary-annular-ring-shaped con vexes 27f from electric corrosion.
The washer 33 has annular-ring-shape as shown in FIG. 5 and is made of stainless steel or phosphor bronze. The washer 33 is provided with a central hole 33a and three fragmentary-annular-ring-shaped lugs 33b radially outwardly protruding from its outer periphery. The washer 33 snugly fits on the cylindrical portion 27d of the hub 27. The shape and size of the fragmentary-annular-ring-shaped lugs 33b coincide with those of the fragmentary-annular-ring-shaped con vexes 27f of the hub 27.
The washer 33 and the hub 27 are assembled so that the fragmentary-annular-ring-shaped lugs 33b abut the fragmentary-annular-ring-shaped convex segments 27f. FIG. 3 shows the assembled condition in normal operation of the compressor 24, wherein the fragmentary-annular-ring-shaped lugs 33b abut the fragmentary-annular-ring-shaped convex segments 27f. 
A washer 34 is disposed to sandwich the flange portion 31a of the inner holder 31 in collaboration with the washer 33. The washer 34 constitutes a second driven friction member. The washer 34 is made of stainless steel and coated with fluororesin coating material to reduce and stabilize the friction coefficient of the abutting surfaces between the flange 31a of the inner holder 31and the washer 34. The fluororesin coating material is an electrical insulating material that protects the washer 34 from electric corrosion.
As shown in FIG. 5, the washer 34 is annular-ring-shaped and provided with a substantially hexagonal hole 34a having a pair of parallel straight edges 34b. The washer 34 fits on the pair of parallel flat chord portions 27e of the hub 27 to rotate integrally with the hub 27.
A disk spring 35 is disposed adjacent the washer 34. As shown in FIG. 5, the disk spring 35 is provided with a central circular hole 35a. The disk spring 35 fits on the cylindrical portion 27c of the hub 27 with its internal edge fitted snugly on the external side surface of the cylindrical portion 27c. 
A nut 36 is disposed adjacent the disk spring 35 to be threaded onto the externally threaded portion 27b of the hub 27.
Fastening force acting on the nut 36 is transmitted to the disk spring 35 to axially compress it. Therefore, fastening force acting on the nut 36 determines the spring force (resilient reaction force) of the disk spring 35. The spring force of the disk spring 35 presses the outer periphery of the disk spring 35 against the outer periphery of the washer 34.
The friction coefficient xcexc1 of the abutting surfaces between the fragmentary-annular-ring-shaped convex segments 27f of the hub 27 and the fragmentary-annular-ring-shaped lugs 33b of the washer 33 is set at a small value of about 0.1.
Micro-irregularities are formed on the surface of the flange 31a of the inner holder 31 opposite the washer 33 by shot blasting or the like to set the friction coefficient xcexc2 of the abutting surfaces between the flange 31a of the inner holder 31 and the fragmentary-annular-ring-shaped lugs 33b of the washer 33 at a large value of about 0.25.
The friction coefficient xcexc3 of the abutting surfaces between the flange 31a of the inner holder 31 and the washer 34 is set at a small value substantially equal to the friction coefficient xcexc1.
The procedure for assembling the power transmission device is as follows. First, the pulley 21, the rotor 22 and the ball bearing 23 are integrated in a unit to be mounted on the cylindrical protrusion 25a of the front housing 25. Then, the hub 27 provided with stopper plate 28 is splined onto the rotary shaft 26 of the compressor 24, whereafter the hub 27 is fixed to the rotary shaft 26 with the bolt 29.
The washer 33 is fitted on the cylindrical portion 27d of the hub 27 with the fragmentary-annular-ring-shaped lugs 33b abutting the fragmentary-annular-ring-shaped con vexes 27f. The outer holder 30, the rubber ring 32 and the inner holder 31 are integrated in a unit to be mounted on the pulley 21, that is, the flange 31a of the inner holder 31 is fitted on the cylindrical portion 27d of the hub 27 and the lugs 30a of the outer holder 30 are fixed to the pulley 21 with the bolts 37.
The washer 34 is fitted on the pair of parallel flat chord portions 27e of the hub 27, the disk spring 35 is fitted on the cylindrical portion 27c of the hub 27, and the nut 36 is threaded onto the externally threaded portion 27b of the hub 27 to a predetermined extent, so that the disk spring 35 is axially compressed to produce a predetermined spring force. The outer periphery of the disk spring 35 is pressed against the outer periphery of the washer 34. Therefore, the washer 34 engages the flange 31a of the inner holder 31 under a predetermined friction force acting on the abutting surfaces, the flange 31a of the inner holder 31 engages the washer 33 under a predetermined friction force acting on the abutting surfaces, and the washer 33 engages the fragmentary-annular-ring-shaped con vexes 27f under a predetermined friction force acting on the abutting surfaces.
After completion of the assembly, the rotation of the inner holder 31 is transmitted to the hub 27 through a first friction engagement path from the flange 31a of the inner holder 31 through the washer 33 to the flange portion 27a of the hub 27 and a second friction engagement path from the flange 31a of the inner holder 31 through the washer 34 to the pair of parallel flat chord portions 27e of the hub 27.
Operation of the compressor 24 will be described. When the pulley 21 rotates, the rotor 22, the outer holder 30, the rubber ring 32 and the inner holder 31 which are integrated with the pulley 21 as a unit also rotate.
Thanks to the spring force of the disk spring 35 caused by fastening force F acting on the nut 36, the washer 34 engages the flange 31a of the inner holder 31 under a predetermined friction force acting between their abutting surfaces, the flange 31a of the inner holder 31 engages the washer 33 under a predetermined friction force acting between their abutting surfaces, and the washer 33 engages the fragmentary-annular-ring-shaped con vexes 27f under a predetermined friction force acting between their abutting surfaces. Therefore, the rotation of the inner holder 31 is transmitted to the hub 27 through a first friction engagement path from the flange 31a of the inner holder 31 through the washer 33 to the flange portion 27a of the hub 27 and a second friction engagement path from the flange 31a of the inner holder 31 through the washer 34 to the pair of parallel flat chord portions 27e of the hub 27. The rotation is further transmitted from the hub 27 to the rotary shaft 26. Thus, the rotation of the pulley 21 is transmitted to the rotary shaft 26 and the compressor 24 operates.
During normal operation of the compressor 24, the rubber ring 32 resiliently deforms depending on the torque fluctuation caused by the load fluctuation on the compressor 24 to absorb the torque fluctuation, thereby reducing the noise of the compressor. Delivered torque T during normal operation of the compressor 24 can be expressed by formula 1.
T=nxc3x97xcexcxc3x97Fxc3x97Rxe2x80x83xe2x80x831
n: number of friction engagement paths
xcexc: friction coefficient of the friction engagement surfaces
F: fastening force acting on the nut 36
R: distance between the friction engagement surfaces and the central axis of rotation
The power transmission device just described is provided with two friction engagement paths. One is through the washer 33 and the other is through the washer 34. Therefore, n=2 is the power transmission device. When n=2, fastening force F acting on the nut 36b can be reduced to half of that when n=1. Thus, the fatigue life of the disk spring 35 is extended and setting of the disk spring 35 is prevented.
When the compressor breaks down and becomes stuck, excessive torque acts on the friction engagement surfaces. The relation between the friction coefficient xcexc1 of the friction engagement between the surfaces of the washer 33 and the fragmentary-annular-ring-shaped convex segments 27f of the hub 27 and the friction coefficient xcexc2 of the friction engagement between the surfaces of the washer 33 and the flange 31a of the inner holder 31 is set to be xcexc2 greater than xcexc1. Therefore, slip occurs first at the friction engagement surfaces of the washer 33 and the fragmentary-annular-ring-shaped convex segments 27f of the hub 27 with smaller friction coefficient xcexc1. Slip occurs also at the friction engagement surfaces of the washer 34 and the flange 31a of the inner holder 31.
When the washer 33 slips on the flange portion 27a of the hub 27 by a predetermined angle in the direction of rotation, the fragmentary-annular-ring-shaped lugs 33b leave the fragmentary-annular-ring-shaped con vexes 27f and the whole body of the washer 33 fits in the concave segment 27g. Thus, the washer 33 moves axially toward the compressor 24 along the cylindrical portion 27d of the hub 27, so that the flange 31a of the inner holder 31 moves axially toward the compressor 24 to abut directly against the fragmentary-annular-ring-shaped con vexes 27f of the hub 27.
The disk spring 35 stretches axially and the spring force of the disk spring 35 rapidly decreases. Therefore, the torque transmission through the aforementioned two friction engagement paths rapidly decreases, the torque transmission to the rotary shaft 26 of the compressor 24 is cut off, and fracture of a belt driving the pulley 21 is prevented.
Once the washer 33 leaves the fragmentary-annular-ring-shaped convex segments 27f of the hub 27, the flange 31a of the inner holder 31 connected to the pulley 21 races relative to the flange portion 27a of the hub 27 and the washer 34. The flange 31a rotates stably between the flange portion 27a and the washer 34, so that excessive noise is not produced. As shown in FIG. 3, the inner holder 31 is located so that a preload biasing the inner holder 31 toward the compressor 24 occurs in the rubber ring 32 when the outer holder 30 is fixed to the pulley 21 with the bolts 37. Therefore, the inner holder 31 moves toward the compressor 24 under the resilient force of the rubber ring 32 immediately after the washer 33 leaves the fragmentary-annular-ring-shaped con vexes 27f of the hub 27. Thus, the power transmission device operates as a torque limiter.
The power transmission device taught by Japanese Patent Laid-Open Publication No.311399/1998 comprises a driving rotary member (inner holder 31) provided with a driving friction member (flange 31a) and a driven rotary member (hub 27) provided with a driven friction member (flange portion 27a). The driving friction member and the driven friction member are disposed in line to each other in the extending direction of the rotary shaft 26. An intermediate friction member (washer 33) is disposed between the driving friction member and the driven friction member to be movable in the extending direction of the rotary shaft 26. The relation between the friction coefficient xcexc1 of the friction engagement between the surfaces of the driven friction member and the intermediate friction member and the friction coefficients xcexc2 of the friction engagement between the surfaces of the driving friction member and the intermediate friction member is set to be xcexc2 greater than xcexc1. The driven friction member is provide with a concave segment 27g in which the intermediate friction member can fit.
The power transmission device of this structure has the following disadvantages.
Even if the initial relation between xcexc2 and xcexc1 is set xcexc2 greater than xcexc1, it may change to xcexc2 less than xcexc1, because of rusting, adherence of oil, etc. If the relation between xcexc2 and xcexc1 becomes xcexc2  less than xcexc1, slip occurs at the friction engagement surfaces of the driving friction member and the intermediate friction member and not at the friction engagement surfaces of the driven friction member and the intermediate friction member. Therefore, the intermediate friction member does not fit in the concave segment 27g of the driven friction member, so that cut-off of torque transmission cannot be reliably achieved. Moreover, machining of the members of the power transmission device becomes complicated and expensive if the relation 112 greater than 111 must be achieved.
Therefore, an object of the present invention is to provide a power transmission device which is free from the defects of the conventional power transmission device, can reliably cut off torque transmission at the time of overload, even if rusting, adherence of oil, etc. occurs, and whose members are simple, easy and inexpensive to machine.
In accordance with the present invention, there is provided a power transmission device comprising a driving rotary member, a driven rotary member fixed to one end of a rotary shaft of a driven device, a first intermediate friction member disposed between the driving rotary member and the driven rotary member to be movable toward the rotary shaft, and a spring for pressing the driving rotary member against the first intermediate friction member, thereby pressing the first intermediate friction member against the driven rotary member, wherein the driving rotary member is provided with a concave segment, the driven rotary member is provided with a convex segment and a concave segment, and the first intermediate friction member is provided with a lug which can fit in the concave segment of the driving rotary member and abut the convex segment of the driven rotary member simultaneously, and also leave the concave segment of the driving rotary member to fit in the concave segment of the driven rotary member, and wherein rotation of the driving rotary member is transmitted to the driven rotary member through the first intermediate friction member with its lug fitting in the concave segment of the driving rotary member and also abutting the convex segment of the driven rotary member during normal operation of the driven device, while when the driven device becomes stuck, the lug of the first intermediate friction member slips off the convex segment of the driven rotary member and leaves the concave segment of the driving rotary member to fit in the concave segment of the driven rotary member, whereby the first intermediate friction member moves toward the rotary shaft of the driven device, pressing force of the spring decreases, and the driving rotary member races.
In accordance with a preferred embodiment of the present invention, the power transmission device further comprises a second intermediate friction member disposed between the driving rotary member and the spring, fitted on the driven rotary member, pressed against the driving rotary member by the spring, and supported by the driven rotary member not to be rotatable relative to the driven rotary member.
In accordance with a preferred embodiment of the present invention, the driving rotary member comprises an outer ring, an internal ring and a rubber ring disposed between the outer ring and the internal ring to be fixed to them.
In accordance with a preferred embodiment of the present invention, the first intermediate friction member is made of ferro-alloy and a magnet is embedded in the concave segment of the driven rotary member.